In recent years, digital circuit technologies such as the LSI (Large Scale Integrated) circuit are used not only for computers or communication-related equipment, but also for home appliances or in-vehicle equipment.
A high-frequency current generated in, for example, an LSI does not stay in the vicinity of the LSI. The high-frequency current widely spreads in the mount circuit board such as a printed-circuit board, inductively couples to the signal wirings and ground wirings, and then leaks from, for example, the signal cables as an electromagnetic wave.
In mixed-signal circuits having both of an analog circuit and a digital circuit, such as a conventional analog circuit a part of which is replaced with a digital circuit or a digital circuit having an analog input/output, one of the serious problems is electromagnetic interference from the digital circuit to the analog circuit.
An effective solution for this problem is to separate the LSI, which is a source of the high-frequency current, from the power sourcing system with respect to the high-frequency current, that is to say, a power-source decoupling technique. A well known noise filter using this power source decoupling technique is a transmission-line type noise filter (Japanese Unexamined Patent Application Publication No. 2004-80773).
This transmission-line type noise filter comprises first and second electric conductors, a dielectric layer, and first and second anodes. Each of the first and second electric conductors is in the shape of a plate. The dielectric layer is disposed between the first and second electric conductors.
The first anode is connected to one end of the first electric conductor in the longitudinal direction, while the second anode is connected to the other end of the first electric conductor in the longitudinal direction. The second electric conductor functions as a cathode for connection to the reference potential. The first electric conductor, the dielectric layer and the second electric conductor constitute a capacitor. The thickness of the first electric conductor is set so as to substantially prevent temperature rise caused by the direct component of the current that flows across the first electric conductor.
The transmission-line type noise filter is connected between a power source and the LSI so as to feed a current from the power source to the LSI through a route formed of the first anode, the first electric conductor and the second anode while attenuating the alternating current generated in the LSI.
As described above, the transmission-line type noise filter constitutes a capacitor and utilizes the first and second electric conductors, which are two electrodes in the capacitor, as transmission lines.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-80773